A, R. Andrew—The Dolgelley Gold-belt. 163 
those of Belt for the Upper Cambrian, as far as they have oeL 
generally accepted. The grouping therefore is— 
Tremadoc Slates 
Dolgelley Beds 
Ffestiniog Beds + Upper Cambrian. 
Visa Eras Pei \ Maentwrog Beds 
Vigra Beds 
Clogau Slates . . Menevian . . Middle Cambrian. 
Gamlan Shales 
Barmouth Grits 
Manganese Shales; Harlech Grits . Lower Cambrian. 
Rhinog Grits 
Cefn Slates 
The order above is in the descending sequence of the beds. Of the 
formations enumerated, those which are more intimately connected 
with the occurrence of the auriferous lodes are the Gamlan, the 
Clogau, the Vigra, the Pen Rhos, and the Ffestiniog. These merit 
more detailed notice. 
Gamlan Beds.—In the north-east part of the district, from the 
neighbourhood of the Afon Gain to near Blaen-y-cwm in the Wnion 
Valley, the lithological break between the Clogau and the Gamlan 
Beds is quite sudden, the black Clogau Slates extending down with 
all their usual characters to a basement grit (the Cefn Coch Grit), and 
the grey and grey-green beds of the Gamlan Group coming immediately 
below. In the western part, from Blaen-y-cwm to Barmouth, there is 
a more or less marked zone of transition, never more than 80 feet in 
thickness, in which there is an apparent commingling of the dark 
shales of the Clogau and the grey, green, and purple bands of the 
Gamlan. It would seem, therefore, that the grit disappears as such 
to the westward: almost everywhere, however, the boundary-line 
between the two types of Gamlan and Clogau Beds can be drawn 
fairly sharply. 
In the north-east country the thickness of the Gamlan Group is 
1200 feet, made up of intermingled grits, greywackes, flags, and 
shales. Following the outcrop of the rocks towards the south-west, 
the grits and greywackes become thinner and thinner, and the 
collective thickness of the beds less and less, until in the neighbourhood 
of the Afon Hirgwm most of the coarse-grained beds have disappeared, 
and the total thickness of the formation has become 750 feet. In this 
vicinity there is a grey flaggy bed near the top of the formation, 
characterized by the occurrence in it of good cubic crystals of pyrites, 
as much as half an inch along the edge. Traced further still to the 
south-west, the grits and greywackes become still finer, and near 
Barmouth the Gamlan Beds are seen to be made up entirely of shales 
and flags. 
The Gamlan Shales contain numerous interbedded sills of greenstone. 
Towards the west, where the group consists chiefly of shales and flags, 
the sills are thinner than they are towards the north-east, where the 
greywackes are more frequent and massive. The massive greywackes 
would offer greater resistance to the folding forces, and would tend to 
form mighty buckles with large cavities between. The softer flags 
and shales would fold and pack more readily, and would not have 
